The present invention is in a process of purifying an exhaust gas which, in addition to dust, contains gaseous pollutants, such as SO.sub.2, SO.sub.3, HCl, HF and nitrogen oxides, by the addition of sorbents to the gas stream and a dry separation of substantially all pollutant-laden sorbents together with the dust.
Such exhaust gases are formed, e.g., in glass-melting processes and in dependence on the nature and composition of the raw materials and fuels employed will contain, per standard cubic meter, 200 to 1000 mg dust, 500 to 3000 mg SO.sub.2 and SO.sub.3, wherein the SO.sub.3 content is 5 to 10%, about 100 mg HCl, 20 to 50 mg HF and 1500 to 3000 mg nitrogen oxides. Cloth filters and electrostatic precipitators are available, inter alia, for collecting the dust. The gaseous pollutants can be separated by the addition of sorbents and reactants, possibly with the aid of catalysts. The added sorbents may be used in dry, wet and quasi-dry processes.
In the dry processes, sorbent powders are added to the gas stream and after a reaction with, or an attachment of, the pollutant, are separated from the gas stream in most cases together with the dust. The separated substances are often re-introduced into the gas stream in order to improve the utilization of the sorbents and to maintain a higher solids concentration in the gas stream so that the probability of a contact between solid particles and pollutant gas molecules is increased. Dry processes afford the advantage that they exert virtually no influence on the temperature of the exhaust gas.
In wet processes the sorbents are introduced into the gas stream as a solution or suspension. While such a process results in an improved reaction and in an improved separation of pollutants, the exhaust gas stream is inevitably cooled to a high degree. Such wet processes involve considerable heat losses and in many cases require a reheat of the purified exhaust gas stream to the lowest temperature which is permissible in the chimney. The wet processes also require an expensive processing of the sorbent liquor.
The quasi-dry processes are a combination of dry and wet processes. In the quasi-dry processes the sorbents are added as a solution or suspension, as in the wet processes, and the rate of liquid is so controlled that the liquid is entirely evaporated in the exhaust gas stream and the pollutant-laden sorbents can be separated in a dry state together with the dusts. In that case the sorbent liquor need not be processed but the temperature of the exhaust gases is greatly reduced so that heat losses occur.
For various reasons, wet process cannot be used to purify exhaust gases emanating from glass-melting tanks and which contain the above-mentioned pollutants. In practice it is particularly undesirable that the sewage must be treated in a plant which is often described as a separate chemical factory, which involves operating costs and gives rise to unpredictable difficulties.
Whereas dry processes can be used for a satisfactory separation of the dusts and of the SO.sub.2 as well as the HF, they do not achieve satisfactory separation of HCl. This relationship is virtually inverted in a certain way as regards the results of the quasi-dry processes, in which all pollutants other than the HF can be separated from the exhaust gas to the prescribed residual concentration with a reasonably low expenditure.
For this reason there is a need for a process for the purification of such exhaust gases which does not have the above-described disadvantages and with which produces a purified exhaust gas containing the pollutants within the legally described limits at an economically justifiable expenditure.